The present invention relates to combustion processes. More specifically, the present invention relates to oxidizing elemental mercury in combustion processes.
The combustion of carbonaceous materials in industrial burners, furnaces and boilers often produces combustion products containing various toxic metals. While many toxic metals are produced in an oxidized form that can be condensed to particulate matter by cooling and filtered out by an appropriate filtration system, many toxic metals such as elemental mercury may remain in vapour form on cooling and can be difficult to collect or filter out of typical combustion gas and flue stack emissions. In the United States alone it is estimated that about 87% of the mercury released to the atmosphere is from man-made sources, and that 33% of this is due to coal-fired utility boilers.
A variety of patents have attempted to address the issue of elemental mercury emission from combustion processes by various means. For example, U.S. Pat. No. 5,695,726 and U.S. Pat. No. 5,569,436 disclose the removal of mercury, cadmium and their compounds from incinerator flue gases by contacting the combustion gasses with dry alkaline material and dry activated carbon followed by solids separation with the resulting gas subjected to quench/wet scrubbing with recycle hydrochloric acid solution formed in situ by absorption of HCl from the gas. Unfortunately, the complicated process depends on the presence of sufficient HCl in the waste combustion gases to react with the mercury metals to yield conversion to the metal chlorides. Thus, fuels that contain little or no HCl may be difficult to treat unless expensive and dangerous chemicals are added to the waste.
U.S. Pat. No. 6,719,828 discloses a high-capacity regenerable sorbent for removal of flue gas and processes and systems for making and using the sorbent. A phyllosilicate substrate acts as a support to a thin layer for a polyvalent metal sulfide. The sorbent is prepared by ion exchange between the silicate substrate material and a solution containing one or more polyvalent metals. Unfortunately, the production of such a regenerable solvent is complex and the regeneration of such a sorbent is time consuming.
U.S. Pat. No. 4,889,698 discloses a process for removal of mercury vapour and other noxious compounds in flue gas from an incinerator plant by suspending powdery activated carbon in the flue gas in connection with a conventional spray absorption process in which the acidic components of the flue gas are neutralized by means of basic absorbants. The cooling of the flue gases in the spray absorption process allegedly provides an efficient adsorption onto the powdery activated carbon.
As compared to elemental mercury which can be difficult to capture from combustion processes, oxidized mercury is known to be removed by scrubbers, particularly wet scrubbers. Accordingly, any combustion process that favors the conversion of elemental mercury to oxidized mercury may reduce the amount of mercury emitted into the environment via flue gases.
There is a need in the art for new methods of combusting fuels such that emissions of heavy metals are minimized. Further, there is a need in the art for new combustion methods that oxidize elemental mercury. Further, there is a need in the art for combustion methods that convert elemental mercury to oxidized mercury, such that the oxidized mercury species can be removed by scrubbers, for example wet scrubbers or the like. Further, there is a need for new methods of combusting fuels that are relatively cheap and that can be performed in existing combustion chambers, boilers, kilns or the like. There is also a need in the art for compositions that can be employed in combustion processes to promote conversion of elemental mercury to oxidized mercury.
The present invention now overcomes the disadvantages of the prior art.